Caesar cipher

The Caesar cipher is the simplest and oldest known type of substitution
cipher, attributed to Julius Caesar who used it to send government messages.
In it, each letter in the alphabet is replaced by another letter using a
predefined rule which shifts the alphabet a uniform amount to the right
or left. For example, a shift of three units to the right, would turn the
"This is secret" into "Wklv lv vhfuhw."